Many publications describe means to monitor the residual charge in batteries and in particular automotive batteries, where the amount of charge remaining may determine whether or not the battery can be used at a later time to drive a starter motor and effectively start an engine.
Also, a discharged lead-acid battery will deteriorate (known as sulfation) rapidly. There are many other applications of batteries where it is desirable to know the amount of residual charge in the battery, for example batteries used for cell phones, or for emergency equipment.
Although modern motor vehicle batteries have progressively improved in capacity there remains a risk that the battery can be accidentally discharged, for example, by leaving the lights on when leaving the vehicle or in other similar ways. Thus the driver on returning to the vehicle will not be able to start the engine because insufficient voltage or capacity remains in the battery to turn the starter motor or to provide sufficient power to the spark plugs, for example, to provide a usable spark. This can of course can be extremely inconvenient for the driver. In some situations, for example, in extreme weather conditions or in localities offering high personal risk a flat battery can be actually dangerous.
Some attempts have been made to overcome this problem, for example, by providing what are in effect double batteries or pairs of batteries but such arrangements may not solve the problem in all circumstances as an opportunity for the starting battery or for the vehicle starting cells to go flat, still remains.
Paredes (U.S. Pat. No. 4,020,414) is one of many inventions that uses battery voltage during discharge as a gauge of capacity, to warn the user of potential failure. These are in effect narrow-range voltmeters for the 11-12 V range, and assume use with a 12 V battery. Paredes uses an array of lamps as a voltage display. Paredes teaches (as in his FIG. 3) that there is a gradual decline of cell voltage as the level of charge is reduced. Stevens (U.S. Pat. No. 4,028,616) includes with his voltmeter a method for measuring voltage under no load and then after a ten-second engine start routine, in order to assess battery condition.
During research for this invention the problem of relying solely on voltage as per the above prior art was realised. It gives too inaccurate a cutoff point.
Sheldrake (U.S. Pat. No. 4,493,001) teaches a motor battery rundown protection system which does include disconnection and reconnection means but this system has only voltage sensing means. The same can be said for Sloan (U.S. Pat. Nos. 4,902,956 and 5,332,958).